If sound travels through matter (air) and gas is a medium then what medium slows down the flow of sound the most?

Vacuum.

The denser the medium, the faster sound travels through it.

Yeah, I think granite is way up there in terms of translating sound waves.

Rubber slows sound down the most to 54 m/s while beryllium speeds it up the most to 12,890 m/s. For reference it’s 343 m/s in air at 20° C. The speed of sound also depends on the medium’s temperature. Sounds would travel slightly faster in warmer air than in colder. More info here.

@ChazMaz: While vacuum does indeed stop (kill) sound, the pedant in me wouldn’t classify it as a medium but as an absence of medium. Sound can’t exist without a medium, so it can’t exist in a vacuum.

@stump: While it is true in extremely general terms, it isn’t true for specifics. Sound travels twice as slow through gold (3,240 m/s) as through aluminum (6,420 m/s) for instance, even if gold is more than 7 times as dense as aluminum.

I would say that the speed of sound depends on the medium’s molecular structure which is again affected by its physical state (gas, liquid, solid, plasma), its temperature, and its density (plus some other factors I likely forgot). So no easy rule of thumb unfortunately.

@utzon I stand (sit, actually) corrected :)

“pedant in me wouldn’t classify it as a medium”

Yea, I was waiting for someone to say that. You get the prize! :-)

@ChazMaz: Hehe